This invention relates to polymer-fiber composites used for the fabrication of decking, railing, siding and structural materials, and more particularly, to foamed composites which are lightweight and provide adequate strength and mechanical properties for building requirements.
Synthetic lumber has been used as a substitute for wood in areas where wood can deteriorate quickly due to environmental conditions. Although in the past, its commercialization was limited by costs, modern recycling techniques and low cost extrusion manufacturing capability have permitted greater penetration by polymer-fiber composite materials into the commercial and residential markets. One such product manufactured under the trademark TREX, by Trex Company, LLC, Winchester, Va., consists of a polyethylene-wood fiber blend which is extruded into board dimensions for decking applications. Polyethylene-wood composite boards in {fraction (5/4)} inch thicknesses have sufficient rigidity to be used as decking planks, but typically are not recommended for structural wood substitutes, such as the lattice structure often used as a support for decks.
Polyethylene composites are attractive because they permit screw fasteners to xe2x80x9ccountersinkxe2x80x9d, such that the heads of the screws bury or at least become flush with the board surface, without predrilling. These synthetic wood products are weather resistant and relatively maintenance free. Once installed, they resist splintering and warping normally associated with wood boards. They are also characterized by xe2x80x9ccolor weatherabilityxe2x80x9d; for example, the TREX product initially is a light coffee brown color and converts to a weathered gray appearance when exposed to rain water and sunlight.
Polyethylene-wood composite boards do not require painting, and never include knots which often result in damage to the surface of ordinary wood lumber, and usually more difficult hammering or screwing of fasteners. These composite materials also do not shed sap, and have a smooth surface texture that is comfortable for even barefoot walking.
In addition to polyethylene, other plastics have been suggested for use in the manufacture of synthetic wood products. Polyvinyl-chloride (xe2x80x9cPVCxe2x80x9d) thermoplastics have been used in combination with wood fibers to make extruded materials, for use in windows and doors, for example. See U.S. Pat. No. 5,486,553 assigned to Andersen Corporation. Such components are designed to substitute for structural wooden members and typically have a tensile or Young""s modulus of about 500,000 psi or greater. Because they are often load bearing, some of these wood fiber-PVC reinforced articles are dense, relatively heavy, and are believed to require predrilling in order to countersink a screw head.
Accordingly, there remains a need for a building material that is light weight, and can permit the countersinking of a screw head without predrilling. There also remains a need for an extrudable polymer-fiber composite that can be tinted in a variety of permanent or semi-permanent colors or to provide a weathered look.
This invention provides foamed polymer-fiber composite building materials which may include about 35-75 wt. % of the polymeric resin, about 25-65 wt. % fiber, and a specific gravity of less than about 1.25 g/cc. The resulting composite includes a plurality of pores or cells therein resulting from the addition of a blowing agent or disbursed gas into a molten precursor of the composite.
The composites of this invention are nearly 10% lighter than non-foamed synthetic boards of similar composition. The preferred vinyl-resin boards are stiffer than polyethylene wood composites of similar thickness. PVC can be foamed through the addition of a blowing agent to a compounded mixture of resin and wood flour. This results in a preferred amount of porosity of at least about a 1% by volume of solids, concentrated primarily in a central region of the cross-section of extruded composite forms made from these mixtures. The tensile and flexural modulus of the preferred board-like members of this invention is less that about 500,000 psi, and generally about 100,000 to 450,000 psi. The resulting board-like surfaces permit the countersinking of screw heads without predrilling.
The polymer-fiber composites of this invention can also include additives for improving the melt strength of a molten precursor of the composite during extrusion operations. The preferred additives for this purpose include acrylic modifiers in amounts ranging from 0.1 to about 15 weight percent. Building materials made from such composites can be tinted to provide a weathered look through the addition of dyes, such as mixed metal oxides and titanium dioxide, pigments, or flyash, for example. In order to reduce costs, larger wood flour particles greater than 30 mesh size can be used.